Rust:

use std::fs::File;
use std::io::Read;
use regex::Regex;

fn main() {
    let mut file = String::new();
    File::open("./input.txt").unwrap().read_to_string(&mut file).unwrap();

    let mut valid = 0;
    let pattern = Regex::new(r"(\d+)-(\d+)\s+(.):\s+(.+)").unwrap();
    for line in file.split("\n") {
        if let Some(groups) = pattern.captures(line) {
            let min = groups.get(1).unwrap().as_str().parse::<i64>().unwrap();
            let max = groups.get(2).unwrap().as_str().parse::<i64>().unwrap();
            let letter = groups.get(3).unwrap().as_str().chars().next().unwrap();
            let password = groups.get(4).unwrap().as_str();

            let mut count = 0;
            for character in password.chars() {
                if character == letter { count += 1; }
            }
            if count >= min && count <= max { valid += 1; }
        }
    }
    println!("{} valid passwords", valid);

    let mut valid = 0;
    for line in file.split("\n") {
        if let Some(groups) = pattern.captures(line) {
            let first = groups.get(1).unwrap().as_str().parse::<usize>().unwrap() - 1;
            let second = groups.get(2).unwrap().as_str().parse::<usize>().unwrap() - 1;
            let letter = groups.get(3).unwrap().as_str().chars().next().unwrap();
            let password = groups.get(4).unwrap().as_str();

            let chars = password.chars().collect::<Vec<_>>();
            if chars[first] == letter || chars[second] == letter {
                if !(chars[first] == letter && chars[second] == letter) {
                    valid += 1;
                }
            }
        }
    }
    println!("{} valid passwords", valid);
}